40 Meters Delta Loop Mechanical Design

Finally the weather broke and I had a few hours to construct and raise the delta loop for 40 meters. Cutting to the chase, it works. However it does need some work, both mechanical and electrical. I'll describe the mechanical design in this article and follow up with the electrical design in a subsequent article.

A delta loop for 40 meters is big. It is in fact much taller than the small tower (Site C) that supports it. This requires a mast to support the apex of the loop which needs to be at least 14 meters above grade so that it doesn't grab anyone's neck as they walk under it. The height constraint is tight since it must also be no higher than 15 meters in order to avoid the regulatory process of both Industry Canada and the city.

Since the tower is 8.8 meters high, the support mast must extend 6 meters above the top of the tower. This is not something that is undertaken without due regard to the wind and bending forces on both mast and tower. The entire system must survive year-round weather events from thunderstorm-caused wind squalls to freezing rain.

The adjacent picture shows the tower, mast, modified TH1vn and delta loop. This is not the final configuration since more work on the mast guying was done.

At full height the TH1vn is up just over 11 meters. While this is not clear in the picture the 4-band dipole runs inside the vertical legs of the delta loop. This is deliberate. I needed that orientation and EZNEC modelling judged the arrangement acceptable. That is, very little interaction between the dipole and loop on all bands of interest. This was confirmed by measurement once it was all in the air.

When I was adding 17 meters to the TH1vn I also took down the mast so that I could prepare the fittings for the delta loop mast. There are two parts to this:

• A second clamp was placed below the TH1vn element/mast clamp for added support and redundancy. Although the dipole doesn't need it (it's very light), the antenna clamp now also supports the weight of the extended mast and the downward force due to the guys pulling on the corners of the loop. It is made of same 1.5" Schedule 40 ABS pipe that couples the TH1vn clamp to the steel (bottom) mast. It is cut on one side and then compressed with a stainless hose clamp.
• Three of the 4' sections of fibreglass mast that I previously used for the experimental 20 meters delta loop form the extended mast. They nest together and slip onto 4" of exposed steel mast above the TH1vn clamp. A shim made of aluminum flashing was cut and formed to allow a snug coupling of the two masts. This is needed since the steel mast is 1.5" O.D. and the fibreglass mast is about 1.6" I.D. The shim is held in place by the pipe clamp that secures the TH1vn to the steel mast.

The snugness of the fit between the masts is important. It must be tight enough to avoid stress points when the delta loop is tensioned but also be easy to drop into place when strapped at the top of the tower and holding vertical the 12' fibreglass mast plus guys and antenna wire above my head to drop it into place. This is the the most dangerous operation in the entire process. I did a test with a single section of fibreglass mast to confirm that I built it properly. Only then did I assemble the extended mast on the ground and lift it into place. It all went more smoothly than I expected, so that was a relief.

Since the fibreglass mast sections are held together only by gravity I had to temporarily secure them in another way during lifting. The rope stay and the vertical legs of the loop were attached to the guy ring, slipped over the mast then pulled tight and secured with a hose clamp to the bottom of the 3 sections. The lift rope was friction fit to both the top and bottom section, with the top one knotted to allow it to be released with one hand.

As I pulled it up to me I slipped the top knot, continued raising it over my head and (holding it very still) dropped it into place. With that done I removed the lift rope and temporary hose clamp and tied the rope guy to one of the tower guys cables. The loop wire had to be cut at the centre of the bottom (horizontal) leg so that one side could be reeled up and dropped over the other side of the TH1vn. All I then needed to do was loosen the mast clamp, raise it to its full height, swing the TH1vn to its final orientation and clamp the mast.

The reason I've gone to the trouble of described this process in detail is to highlight an important requirement of all tower and antenna work: plan everything in advance (and I mean everything!) and test every crucial step on the ground. Too many hams injure or kill themselves by over-confidence, imagining they can resolve any difficulty they run into, then find themselves in poor position or insufficient main strength at a critical step.

Can you really hold that load above your head without it tipping and crashing to the ground and onto you someone else? Try it on the ground, and do it without moving your feet. If you can't or it's iffy you should resolve the difficulty and not take a chance. Trust me, it isn't worth it. Really.

The following are additional notes on the mechanical design. These can apply to any wire antenna.

• Shear pin -- The delta loop is a big antenna that covers a lot of ground. It spans 14 meters over the ground, plus the length of ropes to tie down the corners. As the pictures show there are a number of neighbouring old trees which branches that arch over my property, and the delta loop. A branch that breaks in a storm could fall onto the antenna. The south corner tie-down rope includes a length of synthetic twine to act as a shear pin (a mechanical fuse). Its breaking strength is about 40 lbs (18 kg). The idea is that the twine will break when a tree hits, thus preventing more extensive destruction of the mast and tower.

• Wire selection -- A 40 meters 1λ loop (naturally) requires about 40 meters of wire. In addition to supporting its own weight it must also support the tension needed to reasonably ensure its delta shape. The wire must be up to the mechanical forces. For this antenna I am using 12 AWG insulated soft-drawn stranded copper wire. I would not recommend anything less.

• Coax deflection -- The weight of the coax distorts the shape of the delta loop, as the adjacent picture demonstrates. This can be reduced with higher tension on the loop's tie-down ropes but at the cost of greater stress on the wire and extended mast. The deflection in my case is large since I tried to avoid excess coax droop which would increase coupling of antenna currents onto the coax, and because I used RG-11/U (¼-wave transformer). RG-59 would cause less deflection, if QRO is avoided.

• Mast support -- As mentioned earlier, the extended mast must be guyed. Three-point guying is used. Two of those are the vertical legs plus tie-down ropes, and the third is a rope orthogonal to the loop plane that is tied to the tower. Some care must be taken when setting the tensions of the loop and third rope so that the mast is under sufficient tension, does not deflect from vertical by more than a few inches, and the loop itself is secure. I will need to add a horizontal crossbar to the tower to extend outward the tie point for the rope guy. For the present the rope is tied to the top egg insulator of the most suitable tower guy.

In the next article I'll talk about the antenna itself. As I said, it does work but it requires tuning. Putting it through a tuner to get the SWR down in its present state I was able to work 40 meters DX in Europe and Central America even under poor conditions (October 30 evening) and only the 10 watts from my KX3.

Tuning the TH1vn + 17 Meters Hybrid Trap Dipole

I have gone as far as I will go for the present with multi-band tuning of the TH1vn trap dipole with its 17 meters addition. First I will finish up the discussion of its mechanical design since I have made changes.

The feed point was a week point since the original TH6DXX driven element has nothing more than clamps to attach wires to each half-element. This is suitable for the Hy-Gain BN86 voltage-balun plus beta match on the yagi, but not so much when used standalone as a trap dipole. I replaced my rat's nest of wires with a more structured and robust rat's nest, as shown in the adjacent picture.

As you can see I continue to use pieces of the PVC pipe in additional projects since it is cheap and strong, and sitting in my basement ready for use.

The wires to each half-element clamp run through the pipe to the SO-239 UHF connector. The connector nestles inside the cut-out I made in the pipe centre. The centre pin of the connector fits through a drilled hole in the back of the pipe (not visible in the picture). The connector is screwed onto the pipe, with one screw doing double duty as a post to connect a wire and round lug. It sits diagonally across the pipe since there is no practical way to mount it to the pipe otherwise since the pipe is about the same width as the connector.

The pipe is slightly bent due to the screw tension. While there is no danger of breakage I plan to add a spacer to eliminate any risk of a wire touching the large steel clamp that holds the element halves.

Wires run from the same feed point to hose clamps on the 17 meters element. The stainless steel hose clamps are rated down to ¼-inch pipe (or aluminum rod in this case) but would not work properly without the aluminum shims I added between the clamps and wires. After the picture was taken and initial tuning the solid 12 AWG wires to the 17 meters element were replaced by 14 AWG stranded wire. There was some wire motion in high winds and I want to avoid a mid-winter failure due to metal fatigue.

Here is a somewhat-blurry picture of one half-element of the completed antenna on the tower, sitting 9 meters above ground. You can see how the PVC spacers and plastic ties hold the 17 meters element a constant distance from the trap dipole.

The spacers are secure enough with the antenna mounted and the wind blowing but are prone to rotation when the antenna is moved around during lifting, mounting and tuning procedures. Keeping the aluminum rod directly below the trap dipole is more for aesthetic appearance rather than functional performance. I may eventually snip the ends of the ties.

Tuning the antenna is tricky. The TH6DXX was not designed to accommodate additional parallel dipole. In particular the narrow spacing between the traps for 10 and 15 meters allows little tuning room on 15 meters. To make gross tuning adjustment meant lowering the antenna to the ground. This is easy enough since the tower is short and the antenna lightweight, but would push the spacers out of position as it dragged on the ground or bumped into the tower and guys.

Smaller adjustment I made on the tower by tying the antenna to the side of the tower and accessing the parts of the antenna to be adjusted. This consisted of either the end tip of the trap dipole element or the lateral position of the 17 meters rod.

During this procedure I left the 10 meters tuning alone. It still resonates up in the SSB segment the SWR is a bit higher at the bottom of the band where I primarily operate. Now at 1.6 to 1.7 SWR at 28.0 MHz it work just fine. The other reason I left it alone is that lengthening the inner section of the antenna would lower the resonance on 15 and 20 meters. As I said above, there is little room between the traps to shorten the antenna for operation on 15 meters. This matters since as is now stands the hybrid antenna resonates a little below the band bottom, at about 20.950 MHz. This is fine for CW but the SWR is between 2 and 3 across the SSB segment.

Tuning on 20 meters should be straight-forward since the ends of the element are easily telescoped and have almost no effect on 10 or 15 meters. It did not turn out to be quite that easy.

As I mentioned in the previous article, after adding the 17 meters element the minimum SWR point dropped from 14.0 MHz to 13.7 MHz. Due to the intervening traps this results in a narrow-band SWR response and and SWR above 2 even at 14.0 MHz. The reason for the major impact is circled below.

The end of a dipole is particularly sensitive to the presence of metal, in this case the 17 meters aluminum rod. The same occurs with tuning on 17 but this can be independently adjusted with the wires between the coax and rod. This is why, in general, antennas are shortened with capacity hats and linear loading section connected towards the ends of the dipole elements.

My "toy model" of the hybrid antenna in EZNEC captured the mutual sensitivity at the tips but not the magnitude of the interaction. The model told me to reduce the length of the 20 meters element by 20 cm at each end. I did this and ended up with a resonance at 14.6 MHz! With some quick mental math I decided to try again with 8 cm shortening and the result was right where I wanted it, near 14.1 MHz.

On 17 meters the SWR dipped up around 18.5 MHz but was still broadband enough to work well at 18.068 MHz. Even so, when I spliced in the stranded wire (as described above) I took the opportunity to tune the antenna. It now resonates around 18.2 MHz with an SWR that is low across 17 meters.

In summary, the SWR minimums on 20, 15 and 10 are not far different from what they were before. The main difference is that the SWR bandwidths are somewhat narrower, especially on 20 meters. Plus 17 meters, of course.

Even a dummy load has a 1.0 SWR so this is not an indication of performance. However I can report that it works quite well. Not better than a dipole, but it loads, hears and transmits just fine. It was usable during the tuning -- which was done intermittently over the past week -- and was available for use during the fantastic recent band conditions. I have now rocketed past 150 DXCC countries this year with my QRP plus wires station. The most recent was VU2 over-the-pole on 10 meters, using this antenna.

Since I already have antennas for 10, 15, 17 and 20 meters was this exercise worthwhile? Yes, it was. Apart from the challenge of doing it I do have additional reasons. More on these in a future article.

Mechanical Design to Add 17 Meters to the TH1vn Tri-band Dipole

My feeble modelling attempts to add a parallel 17 meters dipole onto the TH1vn tri-band dipole were not as conclusive as I'd have liked. As I said in that article, one problem is the difficulty of modelling traps, especially those in this Hy-Gain element since they are physically long and I do not know the design parameters. I instead opted to model the element as an inductor-loaded tubing element that would resonate at 14.1 MHz at the measured length of the element.

I did not expect to get the model right but to hopefully get it close enough to identify the key sensitivity parameters, if any, to simply tuning and adjusting. I chose to jump into the actual construction and just see what would happen.

You can see the original antenna feed system I came up with in my first article on adapting the TH6DXX driven element into a simple tri-band dipole. Since the antenna is similar to Hy-Gain's TH1 commercial product I chose to call it the TH1vn.

The physical design and construction started before the modelling work so I was able to get the modified antenna back in the air only one day after my previous article on modelling the design. Before I come to how that went I will take the opportunity in this article to show how I went about building the antenna.

The first thing I needed was a set of spacers (or hangers) to suspend the parallel 17 meters element from the TH1vn. I needed something fast, cheap and reasonably reliable. This is an experiment which does not, at this time, require the effort to make it last for years.

With the ample quantity of PVC pipe left over from construction of the spreaders for the multi-band inverted vee antenna I made the spacers in the same fashion.

As can be seen in the picture there are 8 spacers, 4 for each half of the antenna. Ultraviolet-resistant cable ties thread through the drilled holes to bind the spacers to the TH1vn and 17 meters element. The holes for the TH1vn (on the right) are placed to accommodate the tubing diameters at various points along the antenna. Later I enlarged the holes for the 6 inner spacers to fit longer and wider cable ties.

Notice that the holes for the bottom of the TH1vn are aligned so that the elements are parallel along their lengths. Perfection isn't required for this operation so don't look for precision cutting and drilling!

I then made an error in the material choice for the 17 meters element. Solid copper 12 AWG wire may seem rigid but it isn't. This is amply demonstrated in my first attempt to hang a 4 meters long half-element from the TH1vn.

This is obviously not going to work. Wire in this application can only work if it can be placed under tension. In this application that is difficult to achieve. Note how the element extends beyond the tip of the antenna.

Not having a suitable alternative in my junk box I made a trip to a commercial metals supplier and purchased two 12' lengths of ¼-inch diameter 6061-T6 solid aluminum alloy rod. These are cheap, lightweight and don't dangle too far out the passenger window of my car.

As you can see this worked out far better. However it is not perfect. When the assembled antenna touches an object during the lifting process -- which happens a lot -- the spacers are easily pushed out of position. Fortunately the antenna is very light, which allowed me to move the antenna hand-over-hand at the top of the tower and manually reposition the rods and spacers. As I said, the mechanical design is not perfect.

With a cobbled together feed system -- alligator clip leads, bits of plastic, wire ties, etc. -- I managed to load it up before dark. There is now only a brief window in late afternoon before sunset arrives.

It works, after a fashion. With the same wire lengths connecting the coax to the tri-band element the same as before the SWR minimums moved on each of 20, 15 and 10. What it didn't do is shift in the way I expected.

In the typical fan dipole the antenna for the lowest frequency is relatively unaffected by additional of dipoles for the other bands. That is not what happened. The SWR shifted from the lower end of 20 meters to 13.7 MHz. This is not acceptable, and its under-performance shows. The shift on 15 meters was more modest, having been lowered by about 100 kHz to 21.0 MHz. On 10 meters it shifted upward to 28.6 MHz.

The resonance on 17 meters is off by a little, but will work fine once the other kinks are worked out. Tuning the antenna for this band is relatively easy since it is just the one band. For the tri-band element the tuning will have to be done in stages since there are 3 lengths to be adjusted:

• The section inside of the first trap, including the wire to the coax, affects all 3 bands.
• The section between the traps primarily affects 15 and 20 meters.
• The section beyond the last trap primarily affects 20 meters.

It is that last bit that will need the most adjustment to shift resonance back toward 14.1 MHz.

There is rain forecast for the next few days, which will reduce the time when I can complete this task. Once it's done I'll write a follow-up article on the details of the feed system and how the antenna performs. In this case the performance is all about bandwidth and SWR (impedance) since none of this work changes the far-field pattern. It's still just a multi-band dipole on a short tower.

Modelling Parallel (Fan) Dipoles

I've been remiss on blogging recently due to a combination of travel and other matters. As mentioned in my previous article I had to take close to 2 weeks away from antenna work. As of this long weekend I have restarted antenna work.

I was however able to get in several hours of operating during the great conditions the last few days. The conditions were good enough to put several new countries in the log, even those with large pile-ups which are the bugaboo of QRP operating with zero-gain wire antennas. Examples include: TN, TO2TT (Mayotte), FR, TX5D (Austral I.), ZM90DX (Campbell & Auckland Is.)[no, it's ZL]. As always there are the many that got away, that are just very difficult with my small station.

Now back to the present subject. In an earlier article I showed how fan dipoles are easier to model and reliably build when the antenna wires are parallel rather than radiating outward from the centre. Having established that I went on to build a fan dipole for 30, 20, 17 and 15 (plus 10 and 6) meters. That antenna remains up and continues to work well. It just needs some maintenance to prepare it to survive the winter.

Using EZNEC I modelled the basics of the antenna -- though not all of it -- to test its performance with respect to tuning sensitivity, wire placement, etc. Below is the EZNEC view of the feed point of a parallel fan dipole for 30 (top) and 20 meters, which appeared in that article. Wires #4 and #5 connect the antennas, and the source is at the centre of the 30 meters dipole.

This is not the best model for the feed. The reason is that there are a variety of hidden mathematical quirks in NEC2 that afflict short wires, sharp angles between wires and parallel wires. I did not run into those problems in that particular model but I did in a similar model.

This weekend I took down the TH1vn trap dipole and mast to work on them. The mechanical work to add an extended mast for a 40 meters delta loop was straight-forward. Not so for my other, related project to add 17 meters to the this trap dipole.

It is not easy to model a trap dipole with EZNEC (or other NEC2-based software). The model is nonetheless important since there is a potential for differing interactions on each of 20, 15, and 10 meters. There are both mechanical and electrical challenges which I am currently dealing with. When something comes of this experiment I will report back.

The driven element of the trap dipole (the driven element of my old TH6DXX) is quite short. Each ½-element is 3.8 meters long. This is well short of ¼-wavelength on 20 meters (~5.1 meters) due to the load added by the traps for 10 and 15 meters. Of particular concern to me is that this is shorter than a full-length ½-element for 17 meters (~4 meters).

While this is a rat's nest of issues I chose to concentrate on just one for now, since I expect that to be most messy. This is the coupling of the 17 meters parallel dipole to the full TH1vn on 20 meters. It is generally true that the antenna at the lowest frequency is mostly unaffected by parallel wires for higher frequencies, while the reverse is definitely not true. Right now I need to get the 17 meters design figured out.

I added inductors to the centre of each 20 meters ½-element so that it resonated at 14.1 MHz while at the correct physical length of 3.8 meters. I then added 17 meters wires using the feed as used before (and shown above). EZNEC choked on it, unable to calculate feed point impedance.

After some experimentation I isolated the problem to those short vertical wires. Changing the segment length changed how the problem was manifest but did not make things better.

When you see 2 closely-spaced wires connecting sources and loads in an antenna system, what does that make you think of? It is a transmission line. Transmission lines are much easier to model in EZNEC than all those problematic small wires, so that's what I did.

In the above EZNEC view of the modified feed, the source is placed at the centre of the 20 meters element (wire #1) and a transmission line (red square) connects the centres of both elements. This is certainly a less complex model.

Transmission lines in EZNEC have their limitation since, unlike wires, they do not interact with the rest of the model. They are pure and perfect transmission lines that neither radiate nor absorb RF. For such a short section of line (10 cm in this model) the impact should be negligible. It isn't even necessary to get the nominal impedance of the line exact since it's so short relative to wavelength. I got nearly identical results with impedances from 300 to 600Ω. If you like you can always calculate the actual impedance from the wire diameter and separation.

That's one (big) problem solved. If you model feed systems such as this you may want to do the same. You can find similar parallel dipole feeds in a number of commercial antennas such as the Spiderbeam and HEXbeam.

The other problem is the choice of segment length. While it may be difficult to pick out both of the above pictures, there are green dots showing segment boundaries. In the bottom picture this is ~12 cm for both the 20 and 17 meters elements. This is the important point: the segment lengths should be matched as closely as possible for close spaced wires. Do this for any NEC2-based engine.

Even seemingly small differences can, in the right (or wrong) circumstances, cause significant errors in the modelled results. In some of my models the impact was small and in others it was large. If in doubt, modify the segment count in one of the wires and check the SWR, current and far-field patterns. Or do what I do and always strive to adjust the segment count so that the segment lengths match.

Choosing the transmission line feed model makes this task easier since it is nearly impossible in most cases to also get the segment lengths matched as well with the separate wires in the wired feed model. Sometimes that matters, too, but not always.

To summarize, remember these points in the modelling of parallel dipoles.

• Connect the dipoles with transmission line equivalents of the physical wires. If there more than two dipoles add more transmission lines from each dipole feed point to the next.
• Match the segment lengths in each parallel dipole. Closer matches are possible as you increase the segment count.
• Place the source and transmission line terminations in the centre of each dipole. Do this by selecting an odd number for the segment count and connecting  to the middle segment. When you then place the connection (source or transmission line) 50% from one end of the dipole it will be properly centred.

Once I resolve the mechanical issues with adding that 17 meters element and attempt to tune it to operate on all four bands I will be back.

Overnight DX

This is the season for colds and other virus infections. Such is my plight. One of the downsides is disturbed sleep cycles. But, as they say, when you are given lemons you should make lemonade. So when I found myself wide awake and feeling miserable at 3 AM last night I decided to head to the shack and check out the bands.

There have been some interesting openings this past week due to the equinox and quiet geomagnetic conditions. Much of this passed me by since the bulk of my operating must be in the evening. The polar paths in particular have been intriguing due to the northern route coinciding with the gray line (path along the sunrise or sunset terminator).

When I turned on the radio the very first thing I noticed was the immense quiet. No computers, no lighting and no appliances to spray their debris across the HF bands. It was wonderful. Even the weakest signals were a pleasure to copy.

Since the MUF seemed to fall below 18 MHz the only bands available to me were 20 and 30 meters. I expect the lower bands were also quite good but without antennas for them I did not go there. On these two bands the open propagation paths favoured east and west.

Europeans were scattered here and there. When they did transmit they were quite strong. These were for the most part DXers, not casual operators. That is, they were most likely scanning the bands for attractive DX in the brief period between waking up and heading off to work.

On the other side of the world it was the end of the work day and early evening. VK7CW on Tasmania has a nice strong signal. After losing out to a few Europeans I had a short pleasant QSO with my 10 watts. On this path the inverted vee outperformed the dipole -- likely due to its greater height. While this may sound surprising this was my first VK using my new station. QRP to the other side of the planet requires low competition (from North America at least) and good propagation. This time both were in my favour.

On 30 meters the story was very similar. There were eager Europeans snapping up the available DX in their short morning operating period. As can be seen above, this was shortly after sunrise in western Europe when the low bands experience an enhancement. On 30 meters this can last an hour or two, and gets progressively shorter as one descends in frequency.

After working CY0P the previous evening on 30 meters I was eager for the challenge. The only station that interested me was another VK, this time VK4DX, with a strong signal. He was perfectly workable with my QRP but he was focused on working Europe. It was disappointing but I did not call out of turn. I am sure that if I'd found a frequency and called CQ I could have worked a number of European stations. Except by then I was ready to give sleep another chance and I shut down the station.

In addition to CY0P, the previous evening displayed some good conditions to select areas of southeast Asia. This is evident in the following picture which shows the terminator around the time I was operating.

In particular 9M6XRO had a nice strong signal on 17 meters, even with my simple inverted vee. Notice that 9M is right on the sunrise terminator.

He would have been easy to work if only I had more power. Even so I appreciate the lengthy effort he made to try and pull me through. That's ok, it's simply a challenge to anticipate for another day. I had more success working D2EB, so the evening session was still worthwhile.

Pay attention to the terminator and the path of propagation through areas of light and dark. If you have a small signal similar to you can log some great DX by target times when propagation is most favourable, and when your domestic competition is literally asleep. From time to time step outside your habitual operating pattern. While big stations can "make" DX propagation, little ones should intelligently target advantageous times and frequencies.

Multi-band Inverted Vee Performance

It's been almost 2 weeks since I put up the multi-band inverted vee, so now is a good time to summarize my impressions of its performance. To briefly recap, this is 4-band inverted vee with parallel wires for 30, 20, 17 and 15 meters, and that also resonates on 10 and 6 meters.

The apex is up 14 meters. The heights of the ends depends on the end and the antenna length. The tie-down points are up 6 meters (north end) and 8 meters (south-southwest end). The angles from horizontal are about 40° and 23°, respectively.

The comparison antenna is a tri-band dipole (my so-called TH1vn) up 9 meters during this test. It is oriented to favour NE and SW directions, roughly perpendicular to the inverted vee. With both antennas there are no azimuth gaps in coverage, and in many directions they have similar azimuth gain. The only significant difference is their respective heights.

All contacts were made with 10 watts from my KX3 transceiver. QRP is a good way to show up antenna under-performance. Adding +10 (or +20) db to transmitter power will cover up many antenna problems!

I'll get 6 and 10 out of the way first. I have listened on both bands but did not make any contacts. Nothing heard on 6 meters recently except for ground-wave beacons. On 10 meters there have been openings but these have been in the morning when I had little to no opportunity to operate. Most DX is at very low elevation angles on 10 meters, which should favour the inverted vee (in directions where their azimuth gains are about the same). What I found is that there was no predictable pattern for azimuth and path length: sometimes one one better, yet the other might be better on the same path the next day.

I have some ideas about why this might be happening, pertaining to the likely presence of multiple azimuth lobes in the pattern. Until I model the antenna with EZNEC this will have to remain a guess. That's my best approach since neither antenna is rotatable.

On 15 meters the inverted vee definitely shows an advantage on long paths, such as to KH, ZL and 3D2. The vee's pattern has a notch toward the south and north, and that is noticable in antenna comparisons. Therefore the dipole favours Central America and East/Southeast Asia.

There is no comparison antenna for 17 meters. Even so it works well and has garnered a number of contacts in Europe, Asia and Central and South America. I could heard the South Pacific but not well. The same is true of the Middle East and Africa. This is partly due to conditions.

Apart from azimuth pattern differences, on 20 meters there wasn't too much difference between the antenna. The greater height of the inverted vee made little discernible difference.

I was particularly interested in how it would perform on 30 meters since this is the first proper antenna I have had for this band. As with 17 meters, there is no comparison antenna for 30 meters.

I was reasonably pleased with how it did on 30 meters. The best DX was the omnipresent 3B8CF on Mauritius. Europe, the Caribbean and South America were also easy shots, despite the pattern null toward the south. Since my operating was confined to the evenings I did not test its performance toward the Pacific and Asia. Those are in any case hard nuts to crack on 30 with only 10 watts.

Some stations on 30, strong though they were, were not workable. Either they are alligators -- all mouth and no ears -- or the QRN and QRM at their end defeated my QRP signal.

Overall impressions

This multi-band inverted vee up 14 meters works just as if it were...a multi-band inverted vee up 14 meters. In other words, it works but it's still just an inverted vee. There will be no miracles.

Because the Site-B mast on which it is supported is house-bracketed it is noisier than the dipole on the Site-C tower. This is likely due to all the computers and other electronic appliances in my house and the adjacent house. When the antennas are close in performance I choose the dipole. Atmospheric noise covers up the digital noise on bands below 20 meters, so this is not a factor on 30 meters.

There were many stations on the higher bands -- 17, 15 and 10 meters -- that were barely copyable but not workable. Yet others had no difficulty. These were longer paths, including JY, 9M6, 3D2, HL, DU, KH0 among others. There is no magic sauce that can be added to an inverted vee and QRP that will convert these into QSOs. Even with no one else calling they didn't hear me or copied only a letter or two of my call.

I will have to accept these limitations due to my new status as a little gun or I must contemplate a bigger station. For the present and well into 2014 I will remain a little gun. Afterwards...we'll see.

What this means

I plan to keep this antenna for the winter season. This requires that I do a few things to weather harden it so that it survives the winter, everything from sealing joints to spreader stabilization.

The antenna fills pattern gaps in the dipole, an important consideration when one does not have rotatable antennas. It will also allow me to position the dipole perpendicular to the yet-to-be-installed 40 meters delta loop. I want them perpendicular to avoid potential interactions even though my modelling experiments indicate that interactions are low when they're parallel.

I did not bother adding 40 (or 80) meters to the multi-band inverted vee since my experience is that DX performance will be poor at this modest height. There remains some uncertainty regarding 30 meters, so I am contemplating putting a delta loop on the tower, nested within the 40 meters delta loop. This is added work and mechanical complexity that I'd like to avoid, plus my preliminary models show heavy interaction between the 30 meters loop and the tower. This does not occur to an appreciable degree on 40 meters.

I will start by loading the 40 delta loop on 30 with a tuner to determine whether there is something to be gained with this additional antenna work. Otherwise the inverted vee will be my only 30 meters antenna for the next while.

Further antenna work may be delayed until Thanksgiving (second weekend of October) due to my schedule. That's when I plan to try and turn my tri-band dipole into a quad-band dipole (addition of 17 meters), modify the antenna mount and then build the extension mast to support the 40 meters delta loop.

The weather is great for the present. It won't last much longer so I have to get cracking. Winter is coming.

Raindrops Keep Falling On My Antenna

This past weekend, shortly after erecting and tuning the new multi-band inverted vee, it rained. A lot. Rain affects antennas, sometimes in surprising ways. I'll discuss that and a few more details about the construction of this antenna.

Most antennas don't show much change when it rains. The effects are small enough to escape notice or, if noticed, are not of much concern. Careless design can amplify the effects of rain, resulting in performance problems. The latter is true of my inverted vee.

To understand what can occur we need to talk about velocity factor (VF). That is, the velocity of electromagnetic (EM) radiation is less than c (the speed of light) in any medium other than a vacuum. This matters since the propagation of EM along a conductor is partly in the field surrounding the conductor rather than fully within the conductor.

The VF of air is close enough to 1 that in ham-related applications its effect can be ignored. In coaxial transmission lines the dielectric material filling the space between conductors determines the VF. The fields associated with RF current in both inner and outer conductors is within that material. For example, the most common material, solid polyethylene, causes a VF of about 0.66. That must be accounted for when constructing coaxial transformers.

The same effect occurs with typical commercial ladder line since the wires are encased in plastic and are held apart with plastic spacers. The VF is higher than in coax (in the range of 0.95 to 0.98) since most of the fields travel in air, not plastic.

Antenna wires work the same way. That is why antennas made of insulated wire must be made shorter by ~1% to 3% than indicated by standard length formulas. EZNEC makes this easy to include in models and I always do so for wire antennas I plan to construct.

When it rains there will be some water clinging to the antenna. The EM fields must travel through this water. Therefore when it rains the antenna becomes electrically longer. This is true whether the antenna is made of bare metal or insulated. In most cases the effect is small and can be ignored. Snow has a lesser effect but stays longer.

Freezing rain can have effects on antenna resonance of several percent. However, we are usually more concerned with the antenna staying up and in one piece in this last instance. That tends to distract our attention from high SWR and other performance impacts. If you live in a climate similar to Ottawa where freezing rain is common you'll likely already know what I'm talking about.

In my case the rain caused none of the above effects to any significant degree. It did however cause a major resonance impact on every band. While I did not take the trouble to measure and test, I know what the rain did, and indeed I knew before I raised the antenna. It has to do with wire termination.

Any good antenna book recommends high-quality terminations for wire antennas. A ceramic insulator is best. The ends of antennas are high-impedance (low current, high voltage) points, and are prone to coupling with other conductors in the vicinity. The problem is worse with wires than tubes. With high power, a pointy wire and low humidity you can even get corona discharges to the air itself.

I did not use high-quality terminations for the parallel inverted vee antenna. I would need 8 of them which I did not have and, importantly, would add undesirable weight (and sag). Instead I created end loops out of the wires themselves by tying the bare end back on to the wire. These loops partly compensate for the wire's narrow diameter (lower the terminal impedance). However the problem lay elsewhere: in the nylon ropes.

Ropes contain a lot of air space between the strands. When it rains water fills those spaces. Synthetics such as the nylon rope I used shed water more readily than natural fibres. But that only comes into play after the rain stops, with the nylon drying faster than natural fibre rope. While fresh water is not a great conductor, electrolytes are usually present from collected debris. The high voltage at the ends of the wires amplifies the coupling.

The result in my case is that in the rain the antenna's resonant frequency drops quite a lot on all bands. It averages about 200 kHz. Since my antennas are tuned to favour CW band segments this raised the SWR on much of the phone segments, sometimes well above 3. Since I primarily operate CW this didn't slow me down, at least not after the initial surprise. Some loss due to coupling to the wet rope is also to be expected.

The pattern of a dipole or inverted vee is not appreciably affected by rain, just the resonance and impedance. If you are using a wire array such as a yagi the pattern will be highly distorted. Not only is the driven element affected, so are the parasitic elements. Drop the resonant frequency of a director or reflector by 200 kHz and your front-to-back may entirely vanish. Gain will also drop, though by a smaller factor.

As an example of this effect I've included here the azimuth patterns of a 3-element NBS yagi for 20 meters in free space. One at resonance on 14.150 MHz and one when resonance is lowered by 200 kHz. The latter is simulated by setting the test frequency to 14.350 MHz. While not shown here, the SWR would also rise sharply since the radiation resistance is 2x higher and there is substantial reactance.

As a general rule the tolerances of multi-element arrays are always tighter than in any single element antenna. For this reason I always used ceramic insulators on the wire yagis I built in the past.

The second and last point about the multi-band inverted vee I want to mention is the arrangement of the wires and spreaders. Recall that there are 4 inverted vees in the shape of a virtual cylinder, cut for 30, 20, 17 and 15 meters.

The outermost spreader is placed just inside the termination of the 20 meters antenna. That spreader (and its mate on the other side of the antenna) support wires for 20 and 30 meters, and the ropes coming from the ends of the 17 and 15 meters antennas. To assure the stability of this spreader it is important that the wires for the two lowest bands be positioned opposite each other. Since the nylon ropes run nearly frictionless through the spreader notches (necessary for tension adjustment) the spreader would tip over if those wires were adjacent rather than opposite. The same reason dictates positioning the spreader inside the termination loop of the 20 meters antenna.

Multi-band Inverted Vee

Following on from my previous article I can now report that I have built, erected and tuned the planned multi-band inverted vee antenna on my house-bracketed support mast (Site B). There are parallel inverted vee antenna for 30, 20, 17 and 15 meters. To cut to the chase, yes, it works. In fact it works better than I expected.

The picture shows the antenna as viewed from the backyard, facing approximately northeast. As with the 1.5-band inverted vee experiment the wires are anchored to the north side of the house roof and to the top of the Site-C tower.

A close-framed picture like this is needed to provide a sense of how it's constructed. Now that you've got the finished product in front of you I will step back a few paces and describe how I got here.

In the previous article I compared models of multi-band dipoles (also inverted vee dipoles) using two configurations: radial and parallel wires. The predictability of tuning and performance was significantly better with parallel wires.

If I had only wanted to cover two bands I'd have been done. Because I wanted more bands there are mechanical and electrical challenges with scaling that simple model. Instead of parallel wires I chose a cylindrical model, with antenna legs on the perimeter of a virtual cylinder. There is nothing innovative about this design, which you may have already encountered elsewhere.

Rather than linear spreaders (wire spacers) I needed spreaders that centred the wires equidistant from a common (cylinder) axis. Since my design is for 4 bands I went with an "X" structure for the spreaders. This required fabrication in my workshop. It had to be simple and fast since my time is limited.

There are ample choices of plastic that are suitable for this application: good dielectric properties, strong, easy to work with and resistant to the weather and ultraviolet. For example, Lexan. Wood is not a good choice since it will deteriorate in the weather and can be heavy enough to weigh down the antenna, causing wire sag. For some reason almost all the plastic-supply outlets seem to be on the opposite side of town. To avoid a long drive I decided to first see what I could scrounge nearby.

First I surveyed the inside of the house for derelict plastic that would suit the bill. I struck out so I went browsing through a building materials store. I was going there anyway so this was not a special trip.

Hams are not the only tinkerers. You can almost always find a few middle-aged men casually strolling the aisles of these and similar stores, touching, weighing and even shaking all manner of raw materials. We all have a final picture in mind to which we are trying to fit the available materials. That is, we don't know what we're looking for, but when we find it we will know that it's exactly what we need.

That should explain the following picture. Hopefully you'll find the lettering to be legible.

This is printed on a 10-foot length of ½" PVC water pipe. It's rigid (strong), nonmetallic (dielectric) and sunlight resistant (UV protection). It only cost $3.39, which is guaranteed to be considerably cheaper than almost every alternative. All it needs is a little bit of labour to transform it into antenna gold.

My tools were a tape measure, felt-tipped marker, Workmate, drill, hacksaw and half-round bastard file.

I made the spreader so that no wire was closer than 10 cm to any other. This is not a strict criterion, just enough to ensure against over-coupling caused detuning, even when wind and wire sag lessen the separation.With 4 wires this requires spreader arm lengths of at least 15 cm. I made them 18 cm (7") since the wires penetrate into the arm about 1 cm on each end.

This was not a precision operation! I put the pipe lengthwise into the Workmate and made two cuts along the length of the pipe, dividing it into 4 quarter-round sections. Doing it this way results in wobbly lines and varying widths along each arm. The PVC is thick and strong enough to withstand a few weak points. I could have made them uniform and pretty, if I had taken twice as much time. Friction-fit notches at each end of every arm were made with a hacksaw and then reaming the cut with a drill bit slightly narrower than the wire width.

I used the file to clean the edges, and then to carve a mid-point recess in half the arms. Since the arms have the curvature of the pipe this allows one arm to comfortably nest within its partner. These were then drilled in the centre and screwed together. Since they could still move when forced -- the plastic is very... plastic -- I glued them with cyanoacrylate (super glue). Now they hold their shape.

I made 6 spreaders. There are 3 for each leg: one adjacent to the feed point (see picture below), one just inside the end of the 20 meters leg, and the third one positioned midway between them.

I next proceeded to cut the antenna wires. The 30 meters antenna was the easiest. Since the model showed that the higher-frequency wires would not affect its tuning I simply modeled it by height and interior angle and then cut it about 10 cm longer (5 cm on each side). I made sure to model the insulation covering the wire since it has a significant impact on the wire length. You should typically expect that the wire must be cut about 2% shorter than bare copper. I used #12 stranded wire for 30 meters since it was not only the longest of the four wires but would also be used to set the tension for the other three.

I reused the 20 meters antenna from my 1.5-vee experiment. This was easiest since it was already soldered to the center insulator. However, I knew that it would likely need to be lengthened since, unlike the previous antenna, 20 meters would not be the lowest band. Interaction with the 30 meters antenna would require lengthening the 20 meters antenna.

The single 17 meters leg was similarly reused. The second 17 meters leg and the 15 meters antennas were newly cut. The 20, 17 and 15 meters antennas were made from #14 insulated stranded copper wire.

For modelling geeks only: I did not directly measure the insulation thickness. I looked up the diameters of #14 and #12 stranded copper wire, measured the width of the insulated wire, and subtracted the first number from the second, then divided by 2. The results are approximately 0.4 and 0.5 mm, respectively.

Close up of the fully-constructed feed point, including the first set of wire spreaders

I modelled each antenna as I did for 30 meters. I then mentally added additional length to each, and added several more centimeters for the end loop and wrapping of the wire on the centre insulator wire loop. I hoped this would give me enough wire to allow me to cut and trim each antenna to length. Doing a full model of this antenna would have taken time I do not have right now. I might do so later, long after the fact.

The end of each 30 meters leg is wrapped onto a large galvanized flat washer. Nylon ropes tie the wire-end loops of the other antennas to the same flat washer. Like the wires themselves, the end ropes for the shorter 15 and 17 meters wires attach to the remaining spreaders on the way to this 'terminal' washer.

After soldering the wires to the center insulator the spreaders are placed at their intended positions, with the wires pressed into the spreader notches. The centre insulator is then anchored to some handy fixture in the yard (I used the tower). The tie-down nylon ropes are then pulled to draw tension on the antenna half and tied so that the entire assembly is suspended in midair. The tension for 20, 17 and 15 meters legs is then adjusted so that the wires do not appreciably sag.

Thin nylon rope (I used ⅛") is very stretchy which allows some latitude in setting wire tensions. Just be sure there is no significant slack that wind and gravity can affect. After setting the tension adjust the spreaders so that they are orthogonal to the wires. One or several spreader arms may have moved out of position during this procedure due to the friction fit.

Repeat the above procedure for the other half of the antenna. If, unlike me, you properly machined the spreader notches the wires and rope will not spontaneously self-eject from the spreaders.

This contraption is not too fragile, which is good since it will inevitably need to be dragged along the ground and then bounce against, and tangle with, who know what as it's hoisted by pulley to the top of the mast. There is also the problem of twisting near the terminal washers since the mess of nylon ropes and ends of the 30 meters antenna will twist around each other. This is not really a problem even if it does look ugly. The spreaders keep this from happening to the main body of the antenna.

You can see this happening in the picture below. The flat washer is hidden within that group of knots. It isn't as bad as it looks!

View from the roof of the antenna on the mast, looking towards the southeast

When I was done the antenna worked but was not without faults.

The 20, 17 and 15 meters were still too short, even though I anticipated the problem and compensated for that on 17 and 15. The undershoot was 1.3% on 20 (13 cm), 2.5% on 17 (22 cm) and 1.5% on 15 (10 cm). Wire needed to be added, which I did by tying and soldering additional wire. I did this all one one side of the antenna. There is no need to make the antenna halves exactly equal, but do make sure you are using a coax choke or current balun at the feed point. I worked on the roof (north) side since it was easier to access than the end going to the tower.

When trying out the antenna after lengthening the wires I noticed that on 17 meters the SWR continuously swung back and forth over the range 1.2 and 2.0. It turned out that one leg of that antenna had insufficient tension, causing one section of it to swing close to the 20 meters wire in the breeze. That was adjusted during the next iteration. Unfortunately this was on the tower end so I had to climb the tower, twice, to remove and return the tie-down rope.

The Site-B mast is very flexible. It was challenging to judge the tension from each end of the antenna, particularly on the tower end, so that the mast was not pulled too much to one side. As you can see in the picture the antenna halves are not straight. To do so would require too much tension. It takes a surprising amount of tension to straighten even a light structure such as this antenna. It is about more than aesthetics since the interior angle is more acute due to sag. The performance impact is small, but there is an impact.

After this lengthy description you might not realize how quickly this antenna came together. On day 1 I bought the pipe and then spent no more than 2 hours that evening fabricating the spreaders. On day 2 the wires were cut, the antenna assembled and raised into the air. The on-air performance and SWR were checked that evening, and calculations were done to lengthen the antennas. The third and final day the antennas and rope tension were adjusted, and the antenna was raised into position.

At this point everything worked as planned. I worked stations on every band (to confirm that it did indeed transmit well) and the SWR was below 1.5 on the CW and low phone segments of all four bands. Out of curiosity I briefly tried it on 12, 10 and 6 meters. On 12 it received well but the SWR was very high. On 10 and 6 meters it not only heard well, the SWR was quite low. On 6 it is 1.4 between 50 and 50.2 MHz, and on 10 it is 1.6 at 28 MHz and dips to 1.1 at 28.4 MHz. This goes to prove that if you get enough wire in the air can be unexpected resonances showing up here and there.

I have yet to attempt any QSOs on 10 and 6. It will be interesting to see how it plays on these bonus bands. DX performance is expected to be erratic since dipoles longer than 1 wavelength have multiple smaller lobes, and nulls between those lobes. This is true for both azimuth and elevation, and their number increases as the antenna gets longer.

Once I have used the antenna long enough to assess its DX performance I'll report back. Perhaps within the next week. It should, and so far seems to, show the far field characteristics of an inverted vee on the 4 primary bands.

The apex is up 14 meters, the north end is 6 meters high and the south end is 8 meters high. Its orientation favours south Europe, Africa and Oceania. The current orientation of the TH1vn tri-band dipole fills the gaps in the inverted vee's azimuth pattern. It is currently up 9 meters, having been lowered from 10.5 meters so that I can work on it.

Fall Antenna Work Starts

With the contractors gone I have to catch up to 2 weeks of lost time. That is, time not spent on antenna work. Although work on the antennas wasn't possible I was still able to do computer modelling with EZNEC. I have many ideas brewing in my head, some for implementation this year and some that must wait until next year at the earliest.

In 30 minutes Sunday morning I pulled down the TH1vn tri-band dipole and the 1.5-bands inverted vee. The inverted vees  are down for at least a few days as I put my new plan into play. The TH1vn is now back on the tower but at a lower height where I can easily work on the feed system over the next week or two.

First I'll talk about the TH1vn. As mentioned in the original articles I wrote about this antenna, it is the driven element of my ancient TH6DXX yagi. When I put it up a few months ago I mentioned some odd behaviour on 15 meters. This eventually was discovered to be an intermittent connection...somewhere. I suspected one of the traps for 15 meters, a guess I based on the large effect on that band but lesser effects on 10 and 20.

Doing a search on the web turned up some scary stories of the difficulties of working on these traps, especially those for the driven element. Once I had them in my hands I discovered they came apart quite easily, and that they are straightforward to service. I found no obvious faults. I torqued all the screws, rebuilt the element clamps with stainless steel bolts (the old, rusty ones fell victim to my bolt cutters), and put it back on the tower. It now works as it should.

If the dipole stays problem-free for a few days I will again bring it down for my next experiment, which is to add 17 meters to it. I'll talk more about this in the future after I've made the modification. The modelling for the added band was straight-forward. However I need to consider a lightweight, low-cost and robust mechanical design. The antenna must last for at least the winter. Next year (2014) I may subject it to more modifications for additional performance.

Once the dipole is operational on 20, 17, 15 and 10, it will be time to put up a delta loop for 40 meters. If you've followed along you'll know that I decided on the delta loop as the best of several options for decent DX performance on that band.

Now on to my experiment with the multi-band inverted vee. I chose to do some research and model designs with EZNEC before tackling "cut-and-try" again. Like most hams I used to mostly rely on cut-and-try with dipoles and fan dipoles even though there were instances of deleterious interactions. Since the elements for each band would need to run in the same vertical plane -- the case where interactions are most intense -- I wanted to avoid a lot of physical labour, and frustration.

One thing I did know is that when the elements of a fan dipole (or inverted vee) are maximally separated the interactions are small enough that cut-and-try is a reasonable approach. Some adjustment is usually needed in any case, since even with modelling software there are inevitable interactions with ground, building wiring and so forth. When in the same vertical plane, tuning can be highly sensitive to small changes in wire position.

To this end I performed a couple of modelling experiments. First up is a 30 plus 20 meters fan dipole with a common feed point. In this configuration the 4 wires move out radially from the feed point, in accord with my chosen mechanical feed point design. The model uses dipoles in free space to aoid additional variables. Those can be added to model once a suitable design is selected.

I rotated the 20 meters dipole in 10° increments from 90° (dipoles are orthogonal) to 10°. In the orthogonal start position I "trimmed" the dipole so that the resonant frequencies were 10.125 and 14.100 MHz. The table shows what happens as the 20 meters dipole is rotated.

30 & 20 Meters Fan Dipole – Free Space – Radial fan-out
Angle	30 Meters		20 Meters
	Fr (MHz)	R (Ω)	Fr (MHz)	R (Ω)
90°	10.125	75	14.100	71
80°	10.100	70	14.130	73
70°	10.075	65	14.170	74
60°	10.050	62	14.220	73
50°	10.025	59	14.270	70
40°	10.000	56	14.360	64
30°	9.980	54	14.460	58
20°	9.970	51	14.610	50
10°	9.950	47	14.870	42

Although the affect on the lower frequency antenna is modest, on the higher band the interaction becomes severe as the angle between wires is lowered. This helps to explain the tuning sensitivity. Worse, since the 20 meters dipole would have to be far longer than a naive calculation would give, which make the "cut" part of cut-and-try difficult to put into practice!

There is a better way to construct a fan dipole, one where much of the sensitivity is eliminated. The big hint to me was the long-known technique of using ladder line as a two-band dipole. I do not mean a ladder-line feed system, but ladder-line for the antenna elements. The feed line is coax (with a common mode choke).

You choose a length of ladder line for the lower of the two bands (dipole formula). Then cut the line on one of the leads so that it is the proper length for a dipole on the higher of the two bands. Do this for each leg of the dipole. Tie the two sides of the ladder line together at the feed point, and connect those two to the coax feed line.

The model of the feed point looks something like the adjacent diagram. This is the second model I used for my parallel-wire fan dipole. I then modeled it in EZNEC.

As a starting point I used a 5 cm separation between elements and tuned the antenna for resonance as in the preceding case. Then the separation was increased in 5 cm steps up to 30 cm. To do this I left the horizontal lengths unchanged, only increasing the length of the vertical connecting wires (wires #4 and #5 in the diagram). As before I used free space. The 30 meters dipole is on top, connected to the feed line.

30 & 20 Meters Fan Dipole – Free Space – Parallel fan-out
Separation (cm)	30 Meters		20 Meters
	Fr (MHz)	R (Ω)	Fr (MHz)	R (Ω)
5	10.125	74	14.100	53
10	10.125	76	14.050	59
15	10.125	76	13.980	61
20	10.125	76	13.910	62
25	10.125	75	13.830	63
30	10.125	75	13.760	63

This is much better. Notice how the 30 meters dipole is unaffected by the separation, including any affect from the connecting wires #4 and #5. The impact on the 20 meters dipole is incremental, roughly in proportion to the length of the connecting wires. (Recall that the source is connected to the 30 meters dipole.)

For the modelled feed configuration and range a quick calculation shows that the equivalent length of each 20 meters dipole leg is 0.4x the length of wires #4 and #5 (the separation distance). For example, at a separation of 30 cm the equivalent lengthening of each of the 20 meters legs is 12 cm. I checked this by subtracting 12 cm from each leg of the 20 meters dipole and the antenna model resonated at 14.075. However, the impedance remained at 64 Ω. This is a curious benefit since we do not want the impedance to drop too low or the real antenna, when built, might otherwise have a higher SWR.

You should expect that the length ratio will be slightly different if wires #4 and #5 are a different distance apart. In my model the value is 8 cm (3").

The reason for the lower impedance on 20 meters is that current is induced on the other dipole. This also adds some gain to the antenna, a small 0.8 dBd. The reverse is not true, so the impedance on 30 meters is near to the nominal value of 73Ω in free space and the gain is close to 0 dBd.

This is the model I plan to build for the next iteration of Site-B inverted vees.

1.5 Inverted Vees

In my previous article I described the antenna mast and feed/pulley system I installed at Site-B. As mentioned at the time I did get something in the way of antennas installed on that mast, but did not say what they were. At the time I was in something of a rush, preparing for the arrival of the crew that would be refinishing the exterior of my house.

Since then I have had only a little time to play with the antennas, and no easy way to do further work on them. After a bit of tuning and mechanical adjustments I took the adjacent picture from the roof and then left things alone for a bit.

The wires are all visible in the picture. That you can only see 3 of them is not a mistake. Some explanation is in order.

The wire going down to the left is half of an inverted vee for 20 meters. It is tied to the edge of the roof with a length of nylon rope. The other half of the vee is tied in the same fashion to the top of the tower at Site-C. That's the tower supporting the TH1vn tri-band dipole. The apex of the vee is ~14.1 meters above grade, with south (tower) end ~12 meters up and the north end ~10.2 meters up.

Total length of the antenna is 10.2 meters of insulated #14 stranded copper wire. Its resonant frequency is close to 14.0 MHz with an SWR of 1.2, and it stays below 2 across the band. The average height of the antenna current is approximately 13 meters above ground. This is 2.5 meters (or close to 25%) higher than the dipole. This matters since it is the current that determines the far-field pattern.

The third wire is 4.05 meters of the same wire. Its low point is also 12 meters. It is tied to tower as well, but lower down. Ignore this wire for now since it has negligible effect on 20 meters.

The orientation of the antenna is not ideal. I did what came easy also put the wires as high as possible. The antenna favours east and west directions, and so is -3 db towards Europe and west Asia, and worse towards central Asia. Nominal (modelled) gain in the major lobe is 7.8 dbi at a heading of 90° and elevation of 25°. The pattern is slightly asymmetrical, as expected from their positions. The null to the south is deeper than I'd like: -4 dbi. It is more omnidirectional than a dipole though not by much.

My purpose in putting up this antenna was to test whether a little more height and bending down the legs would outperform the TH1vn tri-band dipole at a height of 10.5 meters. In particular:

• Would the 25% increase in height have a noticable impact on low-angle DX performance. The TH1vn models with a maximum gain of 7.2 dbi at an elevation of 30°, and only negligibly lower at 25°. The antenna favours South Europe, at a compass direction of 70°.
• Would it be more omnidirectional. Wire antennas don't rotate, and I want to cover as many directions as possible, even at the expense of stateside QRM.

If the inverted vee works well enough, and ideally better, I could happily remove the TH1vn and get the low-band antennas installed on the the tower. I have now had some opportunity to compare antennas.

To be brief, the inverted vee works well but is not better than the dipole at the lower height. The EZNEC models appears to reflect my experience with the antennas. In one way this is unfortunate. On the other hand it does mean the modelling can be relied upon, including for future antenna designs at my location.

The highlights of the antenna comparison are as follows:

• On the longest paths (4J, FK8, VK, ZL, JA, BY, etc.) the antennas are roughly equivalent, though not in all cases. The vagaries of multi-path and other propagation effects are noticable even though both antennas are (mostly) the same polarization and in the same vicinity.
• The off-peak orientation of the vee does discriminate against much of Europe, as the model shows. Europe is typically 1 S-unit stronger on the dipole. This is a shame since that is the direction for the bulk of DX paths. However, for some reason northern Europe (TF, OH) slightly favours the inverted vee.
• On polar (north) paths the dipole wins. This includes UA0, BY, VU.
• There is some variation on southern paths, perhaps due to the different positions and depths of the side nulls. Since it can vary by 2 S-units it is a good idea to try both antennas and choose the best one for each station.
• Since the antenna is not up a multiple of ½-wavelenght there is lobe that points straight up. This was confirmed by the relative strength of many mid-distance W and VE stations. This is not desirable.

My current thought is that the inverted vee will continue in some fashion for the winter. The north leg can perhaps be moved to better favour Europe. I have yet to model this. Experimentation will have to wait for at least a week until the work on my house is complete.

Now on to that third leg I mentioned earlier, the final half of this 1.5 inverted vee antenna. I had this odd notion that it might be possible to add a band with only one more wire. If the length is suitably chosen it should form an off-centre fed inverted vee in combination with one, or even both, of the legs of the 20 meters inverted vee.

The reasoning is that any combination of wire legs that has a high impedance on the target band would carry little in the way of antenna current. I decided to try this since it is easier than building a full fan dipole, and I was having some difficulty managing modelled interactions between the wires going to the tower. The closer the wires in a fan dipole come to each other the more they interact and affect the lengths of all antennas except the one for the lowest frequency band.

Since time was short I only modelled the SWR for the third wire, with the aim of making it resonant on 17 meters. I found that the closeness of the legs going to the tower had a large impact on the resonant frequency on 17, but almost no affect on 20. The spread of resonant frequencies went from 17.5 to 18.5 MHz over the range of the tower tie point for the 17 meters leg. You can see how close they are in the photo and model up above.

With that much sensitivity some experimentation is necessary. First, in a quest for height, I tied the rope off at 8 meters height, quite close to the tie point for the 20 meters leg -- the top of the tower is 8.8 meters above ground. That was too close. Then I tried going very low, at the 4.5 meters level. This swung too much in the other direction, much as the model showed. So I split the difference and had a 17 meters antennas that resonated at 18.050 MHz. I decided that was close enough since the band is so narrow. SWR is 1.2 at 18.068 MHz and 1.5 at 18.168 MHz.

I put it on the air and it worked out just fine. Not great, but ok. Stations went into the log. Then I got around to modelling the pattern. That was not fine.

The antennas showed a pretty typical pattern shape for an inverted vee. But something was amiss since the gain in the main lobe was only 3 dbi, several db lower than expected. I then modelled the currents and had a closer look at what was happening.

Imagine my surprise when the image at right appeared! The antenna current chose the adjacent 20 meters leg rather the far one, which I had incorrectly assumed would form the other half of the antenna. Moving the 17 meters legs up and down had little impact on the current in the far 20 meters leg.

This explains the low gain and (as I eventually noticed) the smaller than expected SWR bandwidth. The latter is unimportant on 17, while the former is a problem. I have not investigated further to understand what is happening. Regardless of the reason it seems my "innovation" is a sham. I may have to add bands in a more traditional fashion rather than taking shortcuts. Still, it is in a way quite interesting.

I'll end this article by describing the full extent of the EZNEC model. The model includes all the metal elements in the mast, going all the way to ground level, plus the coaxial cable. The coax is modelled as a wire (the insulated outer conductor of RG-213/U) that is not connected to any other at its ends. To keep things simple this assumes that the air-core coax chokes at the top and bottom are perfect on all bands of interest -- infinite impedance. House metal, in particular the aluminum eaves (my original antenna!) and soffits, are not modelled (at least not yet), nor is the Site-C tower and antennas.

The view of the complete model is shown at right. The perspective is that of an observer to the north-northwest and above the height of the mast. Hopefully it is recognizable.

Antenna currents are small on all these additional conductors on both 20 and 17 meters. I was concerned that the partly-vertical profile of the antenna legs could couple to the mast and coax. For the present I can ignore their small effect.